1. Field of the Invention
This invention relates to compositions for coating drug delivery devices, such as drug eluting vascular stents.
2. Description of the State of the Art
In the field of medical technology, there is frequently a necessity to administer drugs locally. To provide an efficacious concentration to the treatment site, systemic administration of such medication can produce adverse or toxic side effect for the patient. Local delivery is a preferred method in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Thus, local delivery produces fewer side effects and achieves more effective results.
One commonly applied technique for local delivery of a drug is through the use of medicated stents. One method of medicating a stent is with the use of a polymer coating incorporating an active agent. References describe a variety of polymers which can be used to coat stents. Of particular interest is poly(ethylene-co-vinyl alcohol) or EVOH, also known under the trade name EVAL manufactured by EVAL Company of America of Lisle, Ill. EVAL is also distributed commercially by Aldrich Chemical Company of Milwaukee, Wis.
EVAL is a product of hydrolysis of ethylene-vinyl acetate copolymers. Those having ordinary skill in the art of polymer chemistry will understand that EVAL may also be a terpolymer and may include up to 5% (molar) units derived from styrene, propylene and other suitable unsaturated monomers. The EVAL-based stent coating is usually fabricated by preparing a solution of EVAL in a suitable organic solvent or blend of solvents followed by applying the solution onto the stent, for example, by spraying or dipping.
Solvents currently used to prepare the EVAL coating formulation include dimethylacetamide (DMAC) and dimethylsulfoxide (DMSO). These solvents can dissolve EVAL and have been used very effectively to provide an excellent coating. However, their relatively high surface energy, viscosity and degree of wetting of the substrate can lead to a less than optimal coating quality. In addition, the weight of the coating formed per cycle of spraying can be improved resulting in a shorter duration of the coating process.
In view of the foregoing, improvements to the carrying the polymer can be made.